1. Field of the Invention
The present invention relates to a shockwave generator for use in the disintegration of a calculus and, in particular, to a shockwave generator equipped with a piezoelectric element.
2. Description of the Related Art
Recently a calculus disintegrating apparatus utilizing a shockwave has been employed for renal calculus diagnosis. With the apparatus of this type, a shockwave generator is placed outside a patient to radiate a shockwave at a calculus in the region of interest of a human being and to disintegrate it. The shockwave generator utilizes not only a piezoelectric element but also a spark, electromagnetic induction, or explosion energy.
Particular attention has been paid to a shockwave generator utilizing a piezoelectric element (hereinafter referred to as a piezoelectric type shockwave generator) because it can obtain a stable shockwave output at low costs.
The piezoelectric type shockwave generator usually comprises a concave surface type piezoelectric element and a water bag provided on the front side of the piezoelectric element. The piezoelectric element comprises a piezoelectric ceramic plate and a pair of electrodes provided one on each surface of the ceramic plate. The piezoelectric element has a concave surface as an ultrasound radiating surface and a geometrically determined focal point. The water bag holds a liquid medium for transmitting an ultrasound under a better condition in which case an acoustic matching layer is sometimes formed on the surface of the piezoelectric element to allow an acoustic coupling between the piezoelectric element and the water. The acoustic matching layer is normally formed of an epoxy resin. For a diagnostic treatment, the water bag surface is placed in contact with the surface of the patient body. The piezoelectric element, upon being supplied with high voltage, radiates a powerful ultrasound energy. The ultrasound thus radiated converges at the geometrical focal point of the piezoelectric element where a shockwave occurs. With a focal point set at the calculus, a shockwave generated is directed at the calculus to cause the calculus to be broken down.
In the case where the piezoelectric element is in direct contact with water, there occurs a drop in insulation resistance of the piezoelectric element, sometimes causing a short-circuiting across the paired electrodes on both surfaces of the piezoelectric element. Upon occurrence of a short-circuiting across these electrodes, electric current penetrates the patient through the bag water by a high voltage being applied to the patient, offering a "safety problem". Providing the acoustic matching layer on the surface of the piezoelectric element alleviates this problem to some extent. In the use of the shockwave generator for an extended period of time, the piezoelectric element is lowered in its insulation property. Further, the acoustic matching layer is liable to be separated from the piezoelectric element due to the swelling of the water-absorbed epoxy resin and a drop in the strength of its bonding to the piezoelectric element. For this reason, it is necessary to replace the shockwave generator with a new one for a relatively short period of time. A greater maintenance cost is also involved in the conventional apparatus.